The invention relates generally to a method for assessing the quality of a coating process. In particular, the invention relates to a method for assessing quality of a coating process by inspecting a coating formed thereby.
Coatings are commonly employed on various turbine engine components such as airfoils used in power generation as well as aircraft applications. Common coatings include thermal barrier coatings (TBCs), bond coats, and diffusion coatings, for example, and are formed of various materials including ceramics and metal alloys. Prior to scaled production, or a production run, the quality of such coatings is generally inspected to ensure that the coating meets certain requirements, such as microstructure, thickness, and density tolerances. A common technique for inspecting the quality of a coating, and hence the quality of the coating process, is to first coat a sacrificial part and inspect the coating thereof before proceeding with processing a batch of multiple parts.
In the case of turbine engine components, for example, testing is usually carried out by a destructive process, that is, a process which does not leave the part intact and which sacrifices the part under test. Typically, for a turbine airfoil, the blade portion of the airfoil is divided or sectioned into two or more samples, so as to expose cross-sections of the coating. The microstructure and thickness of the coating are then analyzed at several locations along the cross-section to determine whether production with the current processing parameters can proceed.
Typically, the sacrificial airfoil is provided from a pool of defective hardware that was scrapped after either casting or machining. However, particularly for mature hardware designs, scrap hardware can be in limited supply. In such a case, a new part must be produced or purchased for the destructive quality control measurements. The long-term cost of purchasing such parts can be prohibitive. In addition, delays in production can result due to lack of availability of such parts.
While alternative, non-destructive testing processes have been explored for replacing the above-described destructive testing process, it has been shown to be largely unsatisfactory. Attempts to correlate coating thickness distribution statistically along inaccessible regions of a coated part have proved largely unreliable. Attempts at correlating total weight gain of the part do not provide adequate spatial information along the surface contours of the part. Further, particularly for metallic coatings, non-destructive techniques have not yet been developed having requisite accuracy for thickness. For both ceramic and metallic coatings, there are no known non-destructive evaluation methods that adequately characterize the microstructure of the coatings.
Accordingly, a need exists in the art for improved techniques for assessing quality of a coating process, particularly, for coating processes used in connection with turbine engine components under production.
According to one aspect of the invention, a method for assessing quality of a coating process includes the steps of providing a facsimile assembly, the facsimile assembly having a fixture holding an audit coupon, and coating the facsimile assembly to form a coating thereon. Following coating the assembly, the assembly is disassembled, and the coating on the audit coupon is inspected.
According to another aspect of the invention, a turbine engine component facsimile assembly includes a fixture holding an audit coupon, the facsimile assembly replicating a turbine engine component.